As a "gift" to Britain. And what "gift" did Britain give to the Iranians? From [1] SkyNews:

Iran is in the process of freeing the 15 UK sailors and marines taken captive 13 days ago as a "gift" to Britain.

Some have reportedly already been seen by journalists at the presidential palace in the capital Tehran.

Their release was announced by President Mahmoud Ahmadinejad who said they would be handed over to the British embassy.

He said he had pardoned the sailors as a gift to the British people and to mark the birthday of Islam's Prophet Mohammed and Easter....

President Ahmadinejad announced the Britons' release after awarding medals to the "brave" border guards who had arrested them.

"I would like to thank the Iranian coast guard for courgeously defending our Iranian territorial waters," he said....